Plasma fucosyltransferase activity was evaluated as an indicator of an impending blastic transformation in 25 patients with chronic granulocytic leukemia (CGL). Fifteen age-and sex-matched controls were also studied. The level of enzyme activity was significantly higher in the plasma of patients with blastic transformation (1,630 +/- 570 units) compared with steady chronic granulocytic leukemia (509 +/- 110 units) and normal controls (354 +/- 57 units). In three patients with CGL, a rise in fucosyltransferase activity preceded any other clinical or laboratory parameter of blastic transformation by 16-20 weeks.